Midterm 1 Flashcards
what are the layers of connective tissue in skeletal muscle, starting from innermost?
- endomysium
- perimysium
- epimysium
what does endomysium surround?
muscle fibres, which contain bundles of myofibrils (contractile units)
what does perimysium surround?
muscle fascicles, which are made of bundles of multinucleate muscle fibres (myocytes)
what does epimysium surround?
whole muscle, which is made of bundles of fascicles
how are myocytes developed (myogenesis)?
myoblasts fuse to form myotubes (contain a surface satellite cell that makes sure it matures into muscle cell), then undergo terminal differentiation into myocytes (muscle fibres)
where is contractile machinery assembled, and how does it appear?
- myofibrils are assembled in the cytoplasm
- have dark-light banding
how do parallel and pennate muscle fibres differ?
- parallel: lower force, good endurance (ex. sartorius)
- pennate: higher force, lower ROM (ex. rectus femoris) - due to more fibres packed in same volume
why is skeletal muscle striated?
parallel arrangement of myofibrils and highly organized structure of thick and thin filaments
what is the A band?
length of thick filament (myosin) as a whole
what is the I band?
length of thin filament (actin) that hasn’t overlapped with thick filament (myosin)
what is the M line?
the line in the middle of the sarcomere that anchors myosin
what is the Z line?
the line at the ends of the sarcomere that anchor actin
- mark the boundaries of each sarcomere
what is the H zone?
length of thick filament (myosin) that hasn’t overlapped with thin filament (actin)
what are the properties of actin?
- helical coils of g-actin polymerize to form f-actin
- thin filaments construct the cytoskeleton of the muscle fibre
what proteins help stabilize f-actin?
- troponin
- tropomyosin
- nebulin
what are the properties of myosin?
- myosin subunits polymerize in a tail-to-tail formation
- each myosin has a tail region and a cross-bridge region (arm and globular heads)
- globular heads contain light chains important for myosin ATPase activity
what are the components of troponin?
troponin is a trimer:
- TnT: binds tropomyosin
- TnI: binds actin to hold the Tn-tropomyosin complex in place
- TnC: binds Ca2+ (causing a conformational change in TnI)
what do myosin heads contain?
- heavy chain (MHC) - on head
- essential light chain (MLC-1) - closer to head
- regulatory light chain (MLC-2) - farther from head
what is titin?
protein that anchors myosin to Z line
how is the muscle cytoskeleton (f-actin) linked to the ECM?
dystrophin-glycoprotein complex: made up of (transmembrane) sarcoglycans and (membrane-associated) dystrophin (attached to actin)
what causes Duchenne Muscular Atrophy (degenerative muscle disease)?
defects in the dystrophin gene
how do sarcomeres in series vs in parallel differ?
- series: high velocity/ROM orientation
- parallel: high force/low ROM
what are the general components and distinct regions of the NMJ?
2 general components:
- motor neuron
- muscle fibre
3 distinct regions:
- presynaptic region
- synaptic cleft
- postsynaptic region
what composes a motor unit?
one motor neuron, axon, presynaptic terminal (bouton), muscle fibres
- one motor neuron and all the fibres it innervates
what is the innervation ratio?
number of muscle fibres supplied by one motor neuron
how does the innervation ratio vary depending on the muscle?
- if precision is required, less fibres innervated by motor neuron and more individual motor neurons are used
- when force is required, many fibres are innervated by the same motor neuron
what are the functions of presynaptic termini?
synthesis and storage of ACh in vesicles
how is ACh synthesized?
from choline and acetyl-CoA
- requires choline acetyltransferase (ChAT)
how much ACh is stored in a vesicle and where are vesicles stored?
- one quantum (can range from 6000-10000)
- stored in immediate or secondary storage areas for release
what are anatomical considerations of the presynaptic terminal?
- distal nerve is unmyelinated
- P-type Ca2+ channels on presynaptic membrane dominate (facilitate vesicular release)
what are the 2 mechanisms of synaptic release?
- kiss and run
- fusion and collapse: vesicle becomes incorporated into the membrane, releasing its contents
what are the proteins involved in synaptic release?
v-SNAREs, t-SNAREs, synaptotagmin, synaptobrevin
what is the “docking” process?
synaptobrevin (a v-SNARE) and syntaxin/SNAP-25 (two t-SNAREs) form a zippered complex bringing the vesicle and target into proximity (to the active zone)
what is the “priming” process?
complexin primes the complex
what is the “kiss” process?
Ca2+ enters and binds synaptotagmin (Ca2+ sensor); Ca2+-synaptotagmin displaces complexin and binds the SNARE complex causing pore formation (kiss) and release
what is the “run” process?
with Ca2+ depletion, synaptotagmin dissociates, SNARE complex disassembles, vesicle endocytosed
how does tetanus toxin (TNTX) affect synaptic release?
cleaves synaptobrevin, causing rigid paralysis by blocking GABA release in GABAergic neurons (removes inhibitory control of muscle)
- produced by C. tetani
how does botulism toxin (BTX) affect synaptic release?
cleaves synaptobrevin at the NMJ, causing flaccid paralysis
- produced by C. botulinum
what are nicotinic acetylcholine receptors (nAChRs) and how do they work?
postsynaptic membrane protein
- heteromeric pentamer with 2 alpha, 1 beta, 1 delta, and 1 gamma subunit
- 2 molecules of ACh bind (at the terminal end) causing a conformational change that opens the channel allowing Na+ and K+ to move down their electrochemical gradient
- produces an end-plate potential (EPP)
what are mEPPs?
miniature end plate potentials
- due to spontaneous release of vesicles (containing ACh) without APs at presynaptic terminal
- too small to lead to a muscle AP
- used to determine the quantal basis of synaptic release at NMJ (one quantum generates an mEPP)
what events occur at the NMJ?
1) AP in presynaptic motor axon terminal
2) increase in intracellular permeability to Ca2+ through P-type voltage gated calcium channels and influx of Ca2+ into the presynaptic axon terminal
3) release of ACh from synaptic vesicles into synaptic cleft via SNAREs and Ca2+
4) diffusion of ACh to postsynaptic membrane
5) binding of ACh to nAChRs
6) increase permeability of Na+ and K+ causes an EPP
7) depolarization of areas of the muscle membrane adjacent to end plate
8) initiation of AP in muscle
what is dendrotoxin (DTX)?
produced by mamba snake, blocks Kv+ channels on presynaptic terminal (prevents repolarization)
- enhances ACh release
- muscle hyperexcitability
- convulsions
what is alpha-bungarotoxin?
blocks nAChRs
- paralysis, respiratory failure, death at high doses
what is saxitoxin?
produced by algae, blocks neuronal/muscle Nav+ channels
- paralytic shell fish poisoning
- tingling/burning
- shortness of breath
what is tetrodotoxin (TTX)?
produced by pufferfish, blocks Nav+ channels
- loss of sensation
- paralysis of voluntary muscles
- respiratory failure
what is myasthenia gravis?
disorder of neuromuscular transmission characterized by weakness of cranial and skeletal muscles
how does myasthenia gravis affect the NMJ?
- autoantibodies directed against ACh receptors damage postsynaptic NMJ (motor end plate)
- results in impaired neuromuscular transmission: substantial loss of junctional folds (less SA) and reduced # of nAChRs
how does myasthenia gravis present and what are possible treatments?
- diplopia: double vision
- ptosis: drooping upper eyelid
- weakness of facial, bulbar, respiratory, and proximal limb muscles
- acetylcholinesterase inhibitors
- immunosuppressants
where do T-tubules penetrate the the myofibril?
junctions between A and I bands in each half sarcomere
how are muscle APs coupled to muscle contraction?
- upon opening of nAChRs, which results in a passive EPP, Nav+ channels open resulting in a muscle AP
- AP travels down sarcolemma and into T-tubules
- DHPR (L-type calcium channels, Cav1.1) are localized to T-tubules; become depolarized and open -> RYRs (RYR1) are on SR membrane and are physically coupled to DHPRs
- calcium is released from terminal cisternae SR and binds to troponin C
how are DHPRs (Cav1.1) and RYRs (RYR1) coupled, and what is the evolutionary benefit?
voltage-induced conformational change in DHPR induced to RYR, causing calcium release from SR, leading to muscle contraction
- speed
- less extrusion of calcium across the sarcolemma
how are Cav1.1 and RYR1 coupled at the molecular level?
- Cav1.1 grouped into tetrads on T-tubule membrane, aligned and directly opposing 4 subunits of every other RYR1 in the adjacent terminal cisternae
- each subunit of RYR1 has a foot facing the cytosol (each foot complementary to one of the channels in each Ca2+ tetrad)
what are Cav1.1 inhibited by?
L-type calcium channels are inhibited by dihydropyridine (DHP) - used for management of angia (chest pain), cardiac arrhythmias, high BP
- ex. nifedipine: blood vessels; is an antihypertensive (DHP-related)
- ex. verapamil: cardiac; is an anti-arrhythmogenic (non-DHP)
what is responsible for Ca2+ reuptake into the SR?
SERCA pumps against large concentration gradient (1:10000) to bring Ca2+ into SR
- high density of pumps on SR membrane
- high metabolic cost during muscle contraction (active transport)
what is calsequestrin responsible for?
- Ca2+ binds to calsequestrin within terminal cisternae
- high capacity for binding Ca2+
- highly localized beneath triad junction
- aids muscle relaxation by buffering Ca2+, and unloading its Ca2+ near RYR1, facilitating EC coupling
how does Ca2+ facilitate cross-bridge cycling?
Ca2+ released from the SR binds to TnC (4 Ca2+), conformational change causes TnT to pull tropomyosin and TnI out of the way, exposing myosin binding sites on actin
- as long as Ca2+ is present, cross-bridge cycling will occur
how are force and Ca2+ related?
force increases in a sigmoidal manner as intracellular Ca2+ is increased above 0.1 uM
- half-maximal force occurs at 1 uM
how many binding sites does troponin have?
troponin is a Ca2+ sensor - has 4 Ca2+ binding sites:
- 2 have high affinity (bind Mg2+ at rest)
- 2 have low affinity and bind Ca2+ as [Ca2+] rises
why would someone display high body temp and muscle rigidity after receiving muscle relaxants (anesthesia)?
the patient may have Malignant Hyperthermia as a result of a mutation in ryanodine receptors
- anesthetic triggers released of Ca2+ from the SR causing progressive muscle stiffness
- treated with dantrolene (blocks EC coupling in SR)
how is caffeine relevant in skeletal muscle?
can release Ca2+ from SR
what is the sliding filament theory?
cross-bridges that attach between actin and myosin filaments act as independent force generators that produce a contractile force that act to pull the ends of the sarcomere (Z lines) together
- I band changes length only
- sarcomeres contain many cross-bridges that act in parallel to each other so their forces sum
- active force is a result of cross-bridge cycling, displaying an optimal length to which maximal contraction occurs
- displacement of actin is observed in isotonic experiments where force/tension is constant
what are the ingredients for skeletal muscle contraction?
- high myoplasmic [Ca2+]
- supply of ATP
- actin and myosin
- tropomyosin and troponin
what are the states of cross-bridge cycling?
1) attached state
2) released state
3) cocked state
4) cross-bridge state
5) power-stroke state
what is the attached state?
resting state; myosin is attached to actin (ADP released from previous contraction)
what is the released state?
ATP binds to the myosin head, causing myosin to dissociate from actin
what is the cocked state?
ATP is hydrolyzed, causing myosin heads to enter a cocked position (ADP + Pi attached)
what is the cross-bridge state?
a cross-bridge forms and the myosin head binds to a new position on actin
what is the power-stroke state?
Pi is released; myosin head changes conformation, resulting in the power stroke, the filaments slide past eachother
what is muscle tone?
the amount of tension (or resistance to movement) in muscles
what is rigor mortis?
cross-bridge cycle stopping at the attached state due to ATP depletion
how many myosin heads are bound to actin during contraction?
even during maximal force generation, <40% of myosin heads are bound to actin (due to steric hindrance)
what happens to calcium levels during unfused tetanus?
myoplasmic Ca2+ is able to reach to resting between each stimulation
what happens to calcium levels during fused tetanus?
myoplasmic Ca2+ is unable to reach resting between each stimulation, so there is constant Ca2+ release from the SR and therefore constant contraction
what are the different types of myosin heavy chains and what are their differences?
- MHC-I: slow
- MHC-IIa: fast oxidative
- MHC-IIb: fast fatiguable
- different ATPase activity and metabolism
what are the properties of of Type I (slow twitch, endurance) motor units?
nerve:
- diameter: small
- conduction velocity: fast
- excitability: high
muscle cell:
- few fibres
- low force
- oxidative metabolism
- low fatiguability
- slow ATPase rate
what are the properties of Type II (fast twitch, powerful burst) motor units?
nerve:
- diameter: large
- conduction velocity: very fast
- excitability: low
muscle cell:
- many fibres
- large diameter
- high force
- glycolytic metabolism
- fast contraction velocity
- high fatiguability
- fast ATPase rate
- high SERCA capacity
what is an isometric contraction?
when muscle fibres are contracted but remain the same length
- length is constant, force is changing
- experimentally useful in determining the component (force) of contraction that is a result of passive forces (elastic tissue in muscle, titin)
what is an isotonic contraction?
when muscle fibres shorten but the tension remains the same
- force is constant, length is changing
- experimentally useful in determining the displacement of filaments during cross-bridge cycling
what is the optimal resting length for myofibrils?
2.1-2.2 um
what is passive force?
a force that resists stretch slowly at first but rapidly increases once active force decreases
- due to connective tissue and titin (have elastic properties that act like a spring)
how can you measure cross-bridge force experimentally?
- actin filaments attached at each end to polystyrene bead
- optical tweezers (finely focused laser beam) can trap the beam at any specific point and physically move it
- 2 tweezers used to suspend the actin filament above cover glass
- attached to cover glass is a silicone bead with myosin molecules attached
- tension/force held constant with tweezers (ISOTONIC)
- measure displacement of polystyrene bead away from centre of trap
how are motor neurons recruited and why?
from small (type I) to large (type II)
- precision
- energy efficient
- minimizes fatigue
how do the neuron characteristics of different fibre types affect the
characteristics of the muscle and when it is recruited?
- motor neuron innervation of fibre types affects the velocity of shortening
- slower motor neurons (that innervate type I fibres) have smaller diameter and higher resistance -> leads to higher voltage changes for a given input (i.e they will be recruited first when a motor pool is activated)
how much stored ATP does skeletal muscle have?
enough for ~3s of muscle contraction
- can be used immediately
- not energy dense (ineffective store for longer duration exercise)
what high-energy compound is present in skeletal muscle fibres?
phosphocreatine (PCr)
- can be quickly broken down to generate ATP (supplies ~10s of contraction but is regenerated)
- constant source of energy during muscle contraction
- phosphate is transferred to ADP by creatine kinase
how is glycogen synthesized and broken down?
- glycogen synthases add glucose to glycogen
- glycogen phosphorylase removes glucose molecules from glycogen (glycogenolysis)
how is liver glycogen different from muscle glycogen?
- liver has glucose-6-phosphotase , which can facilitate glucose release into blood, raising blood glucose levels (can act as a donor and recipient)
- liver stores 100g of glycogen, muscle stores 400g
what is glycogen used for in muscle?
mobilized during exercise (rather than raising blood glucose)
what is glycogen loading?
increasing carbohydrate intake before an endurance race to build up muscle glycogen
how is ATP generated from glucose anaerobically (O2 not required)?
glycolysis (in cytosol):
- ~12 chemical reactions (i.e. not as quick as creatine phosphate)
- enough energy for ~90s of contraction
- since pyruvate is not being used to form acetyl-CoA for oxidative phosphorylation, it is converted into the byproduct lactate (released into blood) which causes fatigue and muscle soreness
- 2 ATP/glucose
how is ATP generated from glucose aerobically (O2 is required)?
oxidative phosphorylation (cellular respiration):
- slowest route to regenerate ATP
- pyruvate is converted into acetyl-CoA, enters Kreb’s cycle and ETC
- supply ATP for several hours
- occurs in mitochondria
- 36 ATP/glucose
how are FFAs released into blood and how do they enter the muscle cell and the mitochondria?
- FFAs released from adipose tissue by lipolysis circulate blood bound to albumin
- to enter the cell: FFA transport proteins
- to enter mitochondria: carnitine palmityltransferases (CPTs)
how are FFAs used to generate ATP?
aerobic:
- bypass glycolysis and undergo B-oxidation to produce acetyl-CoA for Kreb’s cycle (2 Cs removed each cycle to produce 1 acetyl-CoA)
- also produce FADH2 and NADH
- produces up to 17 ATP (most efficient)
how are AAs used to generate ATP?
last resort; aerobic:
- AAs oxidized after transamination (transfer of amino group to another molecule - must be removed b/c can give rise to ammonia which is toxic)
- bypasses glycolysis; carbon skeletons of AAs enter Kreb’s cycle via conversion into pyruvate and acetyl-CoA
what energy sources are used at rest?
- circulating fatty acids are primary energy source
- O2 is abundant, and aerobic metabolism is used
- glucose taken in and stored as glycogen
- phosphocreatine reserves are built up
what energy sources are used during moderate activity?
- phosphocreatine reserves are used first, but used up quickly
- muscles use aerobic metabolism of fatty acids and glucose released from glycogen stores to make more ATP
what energy sources are used during intense activity?
- muscles lack O2 to support mitochondria
- muscles rely on glycolysis for ATP
- pyruvic acid build up, converted into lactic acid
what are the genetics of mitochondria?
- inherited from mother (oocyte)
- mtDNA is not protected like nuclear DNA (prone to mutation)
- mitochondrial disease often exercise related (fatigue, weakness, exercise intolerance, myopathy) and associated with lactic acidosis (easily fatigued, increases lactate)
how can mitochondrial repair be done through embryo repair?
egg and sperm nuclei within an unhealthy mitochondrial egg are removed and placed into a healthy donor egg after donor’s nucleus has been removed
how can mitochondrial repair be done through egg repair?
mother’s nucleus is removed from the unhealthy mitochondrial egg and is placed within a donor egg that has had its nucleus removed
what signs can signify muscle damage?
- dark, reddish urine -> myoglobin (muscle protein) in the blood that could have leaked out of muscle cell as a result of damage
- elevated creatine kinase -> can leak out of muscle cell as a result of damage
what is submaximal fatigue?
increased effort required to maintain submaximal task
- some fibres become fatigued and less able to generate force (power), additional motor units recruited in order to achieve the same task
how are muscles innervated?
- a single motor neuron can branch and innervated many muscle fibres
- each muscle fibre is innervated by a single alpha motor neuron
- cell bodies of alpha motor neuron efferents are localized to the ventral horn of the SC
what DOES NOT cause fatigue?
depleted energy stores (ex. ATP is always present at sufficient levels)
what generally causes fatigue and how is it related to tetany?
- metabolic byproducts
- during brief periods of tetany, O2 levels are sufficient but force decays rapidly to a level that can be maintained for long periods (subtetany)
- due to rapid failure of fast motor units (recall: powerful bursts but easily fatigued)
- paralleled by depletion of muscle glycogen stores and PCr, accumulation of Pi and lactate
what is central fatigue?
associated with the CNS; at any point:
- premotor cortex, motor cortex, descending pathways, motor neuron activation/inhibition
what is peripheral fatigue?
associated with the function of motor units; at any point:
- NMJ, sarcolemma, T tubules, RYR1, Cav1.1, SR Ca2+ availability, Ca2+ binding and actin-myosin interaction
what might cause central fatigue?
- reduced excitatory drive from the motor cortex
- presynaptic inhibition may be increased via decreased firing of muscle spindles
- 5-HT, NE, DA play roles during exercise
what is the central fatigue hypothesis?
exercise-induced changes in NTs (5-HT, NE, and DA) lead to CNS fatigue
how can central fatigue be measured?
twitch interpolation
- patient instructed to perform a MVC
- partway through, maximum electrical stimulation is applied
- if force is above patients MVC is observed, muscle is capable of generating more force and the poor force during the MVC was due to reduced motor drive from the CNS
how does tetanic fatigue occur?
in 2 phases:
- rapid fall due to fatigue of type II motor units (limited tetanic ability b/c can’t sustain contraction)
- slow decline due to type I motor units (fatigue resistant)
what is tetanic fatigue paralleled by?
- depletion of glycogen and PCr, increased H+ and lactic acid
- decreased pH alters Ca2+ binding to TnC and actin-myosin interactions
- increased Pi affects Ca2+ release, Ca2+ sensitivity, and actin-myosin binding
how can K+ and T-tubules cause fatigue?
K+ pools in T-tubules during high impulse activity:
- high concentrations of K+ causes a decrease in muscle cell efficiency (brings the RMP to be + and depolarized, preventing another AP from occurring b/c Cav1.1 cannot be activated)
- could be prevented by enhancing Na+/K+ pump, but low density in T tubules
how does sustained depolarization of the T-tubules cause peripheral fatigue?
blocks local APs
- change in membrane potential causes a decrease in the release of Ca2+ in the SR (because Cav1.1 are opening less, and in turn RYR1 are opening less)
- leads to decreased contractility
how does impaired Ca2+ release from the SR a cause of peripheral fatigue?
- K+ efflux due to APs cause high [K+]ecf, which reduces voltage sensor activation of Cav1.1 and AP amplitude, reducing RYR1 activity
- ATP in rested muscle is bound to Mg2+ -> use of ATP increases free Mg2+, decreasing RYR1 activity
- increased myoplasmic Pi levels can reduce Ca2+ release by entering SR and chelating Ca2+ (SCl and BCl on SR permeable to Pi)
how does PGC-1a reduce fatigue?
- PGC-1a is a coactivator that is activated by AMPK (phosphorylates it)
- AMPK is activated by CaMKK which is a product of Ca2+ release in skeletal muscle cells
- once PGC-1a is activated, it leads to mitochondrial biogenesis (growth and division of mitochondria) and lactate metabolism
how does are muscle fibres affected by the nerves that innervate them?
- fibres innervated with type I motor neurons become slow oxidative MUs
- fibres innervated with type II motor neurons become fast MUs
how are muscles lengthened during growth and what does it do?
results from the formation of additional sarcomeres
- reversible (shortening occurs with immobilization)
- does not change force but increases velocity and shortening capacity (in series)
how does strength training affect muscles?
increases in strength and diameter due to hypertrophy
- increasing diameter of myofibril increases force but not velocity or shortening capacity (in parallel)
why does muscle atrophy occur and how?
- when muscles aren’t used (required to maintain growth and development)
- must require a load or else atrophy occurs
- inhibition of protein synthesis
- stimulation of protein degradation
how does testosterone display myotrophic (anabolic) activity?
anabolic steroids increase muscle mass
what pathways regulate muscle atrophy?
PI3K and AKT are deactivated which activates FOXO:
- FOXO: transcription factor, activates atrophy-related genes (ex. atrogin: ubiquitin ligase; when ubiquitin is activated is activated it makes proteins for destruction and is responsible for myocyte protein degradation)
- when muscle tension is reduced (due to disuse), MuRF (ubiquitin ligase) dissociates
what mechanisms contrast muscle atrophy and are active during exercise?
- increased mitochondrial density (due to PGC-1a regulating mitochondria)
- increased oxidative capacity
- angiogenesis: increased capillary density
how are male hormones normally produced?
- LH acts on Leydig cells (in testes) that produce T
- FSH acts on Sertoli cells (in testes) that produce Androgen Binding Protein (ABP) which concentrates T in seminiferous tubules
how do anabolic steroids cause hormone disturbances?
- enhance negative feedback resulting in lower LH levels, reduced T levels and lower intracellular T
- enhanced muscle mass but smaller testes and reduced spermatogenesis
how can reinnervation change a muscle fibre type?
- if muscle is stimulated at a low frequency, will become slow (type I)
- if muscle is stimulated at a high frequency, will become fast (type II)
what are characteristics of tendons?
- resists high tensile force
- flexible so it can bend at joints
- shock absorption
- transmits muscle force to skeleton
- stretches and stores mechanical energy
what is the difference between external and internal tendons?
- external tendon: extends from muscle belly
- internal tendon: extends into muscle belly
what is an aponeurosis?
extensions of internal tendons on pennate muscles (2 aponeuroses - muscle fibres join the 2 and each thins into a small tendon that is attached to bone)
- stretched by forces of muscle contraction
- absorbs energy (like a spring) and return it when they recoil
what are the functions of a tendon?
- mechanical energy is conserved (ex. during running)
- can amplify muscle power (ex. preparing for a jump by bending leg, loading tendon through muscle contraction)
- a rapid decline in mechanical energy can be stored as elastic strain (reduce peak power input) (ex. deceleration)
what is pennation/pennation angle?
the organization of muscle fibres at an angle to the line of action, which facilitates higher torque/force output
- when the muscle contracts, the pennation angle increases, which slows contraction
what are the trade-offs of muscle pennation?
as muscle force is increasing due to a greater pennation angle and slower contraction, the efficiency of contraction is decreasing as the fibres are moving farther away from the line of action and reduced shortening velocity
how do eccentric (lengthening) contractions produce force and why do they use less ATP?
- don’t need to break and re-establish myosin-actin cross-bridges
- these interactions can absorb some force from the load, meaning they can break and reform at no energy cost
why do concentric (shortening) contractions produce less force?
require myosin heads to be attached more of the time, producing less force
what is the physiological cross-sectional area (PCSA) and why is it useful?
- cross-sectional area of a muscle at a given point (usually widest point) perpendicular to its muscle fibres
- helpful in estimating the number of sarcomeres in parallel and force generation
what are muscle spindles?
muscle receptors made of intrafusal muscle fibres (bag and chain) that detect changes in muscle length
- lie in parallel with regular muscle fibres
- has sensory (Ia and II afferents) and motor (gamma efferents) innervation
what are group Ia afferents responsible for?
sensitive to change (dynamic)
- fire when stretch occurs, not release
- innervate b1, b2 and chain fibres
what are group II afferents responsible for?
sensitive to the length or duration of the change (static)
- increase firing during stretch
- decrease firing during release
- innervate b2 and chain fibres
what is the dynamic gamma end?
- on b1 fibres
- response of Ia is enhanced but II remains unchanged
- activated during conscious proprioception (on a bus trying to stay upright, maintaining balance)
what is the static gamma end?
- on b2 and chain fibres
- sensitivity of type Ia and II is increased
- active during unconscious proprioception (swaying while standing)
how do gamma motor neurons regulate spindle sensitivity?
alpha-gamma coactivation
- when a muscle spindle contracts, the spindle shortens and become “unloaded”, stops firing and becomes insensitive to further change
- gamma stimulation counters this: intrafusal fibres shorten along with extrafusal fibres thus maintain their sensitivity
what are golgi tendon organs (GTOs)?
muscle receptors that monitor muscle force
- only contain Ib afferents (no efferents)
- in series with muscle
- force is proportional to stretch
